Expt 5 Common Ion Effect

EXPERIMENT NO. 5
COMMON ION EFFECT

Lopez, Pauline G. HIJ-2, Group No. 5, Ms. Jervee M. Punzalan
Ramales, Ayane Mark Q. September 15, 2014

I. ABSTRACT

The common ion effect involves how the solubility of a salt changes when some ion that is common to both added substance and the salt in question is introduced. The presence of a common ion hinders the ionization of a weak acid or a weak base hence this is known as the common ion effect. Simply put, the effect of the common ion is to decrease the solubility of the salt. Reactions involving weak conjugate acid-pairs can resist massive change when small amounts of strong acids and bases are added to it. Buffer solutions are also related with the common ion effect. A buffer is an aqueous solution composed of a weak acid and its conjugate base or a weak base with a conjugate acid that will have a minimal change in pH when a strong base or acid is added to it. The results and observations of this experiment proved (a) the occurrence of the common ion effect through the determination of pH and titration and (b) how minimal the change in the pH of a buffer solution is upon the addition of a strong base and a strong acid.

II. KEYWORDS: Common Ion Effect, Buffering Effect, Buffer Solutions, Ionic Equilibrium, Solubility, pH
III. INTRODUCTION
The process in which an atom or a molecule is converted to an ion by the addition or removal of a charged particle is called as ionization. Substances the undergo ionization reaches ionic equilibrium where the concentration of the reactants and the products don’t change over time and the rate of the forward reaction is equal to the rate of the backward reaction. Adding a common ion to the mixture in equilibrium will make the reaction experience what is known as the common ion effect. When the common ion effect occurs, there will be a shift in equilibrium caused by the addition of a compound having a common ion with the substance. Faster rate of the backward reaction will happen to counterbalance and to re-establish the equilibrium of the solution.
Buffers or buffer solutions are used as a means of keeping the pH at a nearly constant value. Solutions are considered to be buffers if they have a minimal change in pH upon the addition of an acid or a base.
The solubility of a substance is decreased by the common ion effect. Based on Le Chatelier’s principle, an addition of another ion can be recognized as a stress applied to the system thus making the reaction proceed in a way to bring back the equilibrium. By adding common ions, the reaction will proceed in a way where more solids or precipitate will be made therefore, decreasing the solubility of the solute present in the solution.
In this experiment, the effect of the presence of the common ions will be determined regarding the ionization of the acids and bases, buffer solutions and its effect on solubility.
IV. EXPERIMENTAL
The experiment was divided into three parts. The first part involved the effect on the ionization of acids and bases. The following solutions were prepared for this experiment:
10 mL 0.1M HCl + 2 mL H2O
10 mL 0.1M HCl + 2 mL 0.1M NaCl
10 mL 0.1M HAc + 2 mL H2O
10 mL 0.1M HOAc + 2 mL 0.1M NaOAc
10 mL 0.1M NaOH + 2 mL H2O
10 mL 0.1M NaOH + 2 mL 0.1M NaCl

The pH of each of the solutions were then determined and recorded.
Observation of buffer solutions is the objective of the second part of the experiment. First, the pH of distilled water was determined. Two test tubes were then each filled with 10 mL of distilled water and a single drop of 6M HCl was added to the first test tube and one drop of 6M NaOH was added onto the second test tube. The pH of both of the solutions were determined afterwards.
In this part of the experiment, the following solutions were also prepared:
10 mL 0.5M HOAc + 10 mL 0.5M NaOAc
10 mL 0.5M HCl + 10 mL 0.5M NaCl
10 mL 0.5M HNO3 + 10 mL 0.5M NaNO3
10 mL 0.5M NaH2PO4 + 10 mL 0.5M NaHPO4
10 mL 0.5M NH4OH + 10 mL 0.5M NH4Cl
The pH of each of the solutions were then measured and recorded and each of the solutions was divided into two equal parts. One part was dropped with 6M HCl and to the other part was dropped with 6M NaOH. The pH of each resulting solution was measured.

Lastly, the effect of the common ion on the solubility of slightly soluble salts was observed in this part of the experiment. In a 100-mL beaker, 50 mL of distilled water was poured in and mixed with 0.5g of sodium benzoate. The mixture was heated to about 40oC.
Benzoic acid crystals were then added to the hot solution until the crystals no longer dissolve. The solution was cooled to room temperature with stirring before it was filtered. 10 mL of the filtrate was transferred to a 50-mL Erlenmeyer flask.
The filtrate in the 50-mL Erlenmeyer flask was then added with 2 drops of phenolphthalein and was titrated with 0.01M NaOH to a light pink end-point.
V. RESULTS
Part A. Effect on the Ionization of Acids/Bases
Table 1-a. Measured pH of the Solutions
Reagents
pH
10 mL 0.1M HCl + 2 mL H2O
1.34
10 mL 0.1M HCl + 2 mL 0.1M NaCl
1.26
10 mL 0.1M HAc + 2 mL H2O
2.37
10 mL 0.1M HOAc + 2 mL 0.1M NaOAc
4.50
10 mL 0.1M NaOH + 2 mL H2O
11.28
10 mL 0.1M NaOH + 2 mL 0.1M NaCl
11.24

Table 1-a shows the respective pH of each of the solutions prepared in Part A

Part B. Buffering Effect
Table 2-a. Measured pH of the Solutions Before and After Addition of Acid and Base
Solution
pHorig pH+HCl pH+NaOH
Conclusion
(theoretical)
A
4.60
4.49
4.72
Buffer
B
0.86
0.74
0.96
Not buffer
C
0.89
0.73
0.84
Not buffer
D
4.21
4.44
5.56
Buffer
E
8.2
7.63
8.33
Buffer

Table 2-a identifies the classification of whether the solution exhibits a buffering property or not. It also shows the original pH of the solution and its pH after the addition of a strong acid and a strong base.
Part C. Effect of Common Ion on the Solubility of Slightly Soluble Salts
Volume of 0.01M NaOH: 25 mL
Solubility of benzoic acid in water: 0.0074 M
Solubiity of benzoic acid in sodium benzoate solution: 0.1326 M
VI. DISCUSSION
Part A. Effect on the Ionization of Acids/Bases
This part of the experiment focuses on the effect of the presence of common ion in the ionization of a weak acid or a weak base. The results showed that the solutions 0.1M HCl – H2O and 0.1M HCl – 0.1M NaCl possessed a relatively low pH. This is due to the presence of HCl in the solution that is known to be a strong acid. HCl ionizes as follows:
HCl  H+ + Cl-
Theoretically, the pH of the second solution should be higher than the first due to the presence of additional Cl- ions from the NaCl hence shifting the equilibrium in contrary to dissociation and reducing the concentration of H+. However, the difference in pH of the solutions is only minimal and they still have an almost equal value of pH.
On the other hand, the third and fourth solutions – 10 mL 0.1M HAc + 2 mL H2O and 10 mL 0.1M HOAc + 2 mL 0.1M NaOAc – should theoretically have a pH that are relatively close to each other. The ionization process for HOAc is:
HOAc H+ + OAc-
The addition of OAc- ions from NaOAc caused the equilibrium to shift to the left therefore decreasing the concentration of H+ and increase the pH. The results showed that the fourth solution has a pH value higher that that of the third solution proving that the common ion has an effect to the ionization of HOAc.
Lastly, the two remaining solutions – 10 mL 0.1M NaOH + 2 mL H2O and 10 mL 0.1M NaOH + 2 mL 0.1M NaCl – involve the same principle. NaOH is a strong base and it dissociates completely in a solution. Thus, it would not be greatly affected by the addition of other ions. This conclusion is strongly supported by the pH values of the last two solutions having a very close pH values.
Part B. Buffering Effect The second part of the experiment aims to determine the effect of buffer solutions. In this experiment, the results showed that solutions A, B and C possess buffer properties. Theoretically speaking, B and C should not be considered as buffers since the reagents mixed to prepare solutions B and C do not have the enough strength to cause a neutralization reaction between the two reagents.
However, the results obtained from solution A proved that it is a buffer solution. Aside from the obtained results, solution A is considered as a buffer because this solution is considered as a conjugate acid-base pair that should exhibit a buffer effect.
Solutions D and E, based from the data obtained, are not considered as buffers. Theoretically, these solutions should possess a buffer property since they are conjugate acid-base pairs that exhibit buffing effects.
Part C. Effect of Common Ion on the Solubility of Slightly Soluble Salts The last part of the experiment showed that the addition of a second solute that has a common ion to the prepared system decreases the solubility of the first solvent, which is a slightly soluble salt. In this experiment it is C6H5COO­-, reduced the solubility of the sodium benzoate. This results to a backward shift to the solution.

VII. CONCLUSION
The presence of a common ion in a reaction will yield an effect on the ionization of acids and bases, a buffering effect and an effect on the solubility of slightly soluble salts.

The presence of a common ion will either increase or decrease the pH of an acid or base during ionization.

A buffer effect, wherein the common ion acts as cushion that prevents the significant change on pH of either weak acid or a weak base. This is because the common ion consumes the strong acid or strong base thus preventing any significant changes in the pH. The ionic equilibrium is reestablished by means of a forward shift.

Theoretically, the addition of a common ion causes a decrease in the solubility of a slightly soluble salt.

VIII. RECOMMENDATIONS
For better results, it is recommended to make sure that the pH meter to be used is calibrated and thoroughly cleaned before and after use. It is also recommended to ensure the purity of the substances to be used because this would greatly affect the results of the experiment.
IX. REFERENCES
Chang, R. (2010). Chemistry (10th ed.). New York: The McGraw Hill Companies, Inc.
Brown, Theodore E., Lemay, Eugene H.,Bursten,Bruce E.,Murphy, Catherine (2008). Chemistry: The Central Science (11th ed.), New York: Prentice Hall, ISBN 0136006175
The Common Ion Effect (n.d.). Retrieved from http://faculty.ncc.edu/LinkClick.aspx?fileticket=KFQ8mVJpEPY%3D&tabid=1904

I hereby certify that I have given substantial contribution to this report.

________________________ Pauline G. Lopez

________________________ Ayane Mark Q. Ramales

References: Chang, R. (2010). Chemistry (10th ed.). New York: The McGraw Hill Companies, Inc. Brown, Theodore E., Lemay, Eugene H.,Bursten,Bruce E.,Murphy, Catherine (2008). Chemistry: The Central Science (11th ed.), New York: Prentice Hall, ISBN 0136006175 The Common Ion Effect (n.d.). Retrieved from http://faculty.ncc.edu/LinkClick.aspx?fileticket=KFQ8mVJpEPY%3D&tabid=1904 I hereby certify that I have given substantial contribution to this report. ________________________ Pauline G. Lopez ________________________ Ayane Mark Q. Ramales